Mononuclear phagocytes are closely associated with diseases of the central nervous system. Microglia found in normal adult brain are highly ramified, quiescent cells that retract processes and become reactive during CNS injury (Rio-Hortega, 1932). Reactive microglia (activated brain mononuclear phagocytes) have been identified with Alzheimer Disease (AD) neuritic plaques (Bolsi, 1927; McGeer et al., 1987; Rogers et al., 1988; Giulian, 1992; Perlmutter et al., 1992; Giulian et al., 1995a). As a result, .beta. amyloid (A.beta.)-induced neuron damage is thought to involve inflammatory cells. In Alzheimer Disease, quantitative histopathology has determined that &gt;80% of core plaques are associated with clusters of reactive microglia while fewer than 2% of diffuse A.beta. deposits show such an association (Giulian et al., 1995a). These observations suggest that brain inflammatory responses may be directed specifically against the constituents of neuritic and core plaques. As the principal immune effector cells of the brain, activated microglia are capable of releasing such cytotoxic agents as proteolytic enzymes, cytokines, complement proteins, reactive oxygen intermediates, NMDA-like toxins, and nitric oxide (Thery et al., 1991; Giulian, 1992; Rogers et al., 1992; Lees, 1993, Banati, R. B., 1993).
Alzheimer Disease accounts for more than 15 million cases worldwide and is the most frequent cause of dementia in the elderly (Terry, R. D., Katzman, Bick, K. L. (eds), 1994) and is thought to involve mechanisms which destroy neurons and synaptic connections. The neuropathology of this disorder includes formation of senile plaques which contain aggregates of A.beta.1-42 (Selkoe, D. J., 1991, Yankner and Mesulam, 1991; Price et al., 1992; Younkin, 1995). Senile plaques found within the gray matter of AD patients are in contact with reactive microglia and are associated with neuron damage (Terry, R. D., 1994 a and b, Masliah, E., et al., 1994, and Perlmutter, et al., 1992). Plaque components from microglial interactions with A.beta. plaques tested in vitro were found to stimulate microglia to release a potent neurotoxic amine, thus linking reactive microgliosis with AD neuronal pathology (Giulian, et al., 1995). However, the plaque component, or components, which elicits neurotoxic responses in microglia remained elusive.
A second type of A.beta. accumulation found in both AD and aged normal brain consists of diffuse plaques (discrete mesh-like structures of 70 to 100 .mu.m diameter, visualized by silver staining, thioflavine S, or immunohistochemistry) which are not associated with such pathological changes as dystrophic neurites or decline in cognitive function (Yamaguchi et al., 1988; Masliah et al., 1990, 1993). Finally, diffuse, amorphous deposits of A.beta., demonstrable only by immunohistochemistry, have been described in aged brain and as an early manifestation of AD-like pathology in Down's syndrome (Giaccone et al., 1989; Verga et al., 1989). Although the mechanisms which link neuritic and core plaques to dementia remain unresolved, two principal hypotheses have been advanced: 1) that AD acts as a potent and direct neurotoxic agent (Yankner et al., 1990) or 2) that neuritic/core plaques elicit a cascade of cellular events which lead to neuronal pathology (Davies, 1994; Giulian et al., 1995a). Support for the first hypothesis comes from in vitro observations in which synthetic A.beta. peptides appear toxic to enriched cultures of neurons (Pike et al., 1991; Cotman et al., 1992) or to various non-neuronal cell lines (Behl et al., 1994; Pollack et al., 1995). Support for the second hypothesis comes from evidence that neuritic/core plaques are not directly neurotoxic, as shown by the fact that neurons can be grown successfully atop A.beta. peptides (Koo et al., 1993; Wujek et al., 1996), that neuritic/core plaques added directly to neurons do not cause neuron damage (Giulian et al., 1995a), and that A.beta. peptides infused into the brain do not cause tissue injury (Games et al. 1992; Podlisny et al., 1992; Stephenson and Clemens., 1992).
Since the description by Bolsi (1927) of reactive microglia near plaques in AD brain, it has been uncertain whether these reactive non-neuronal cells actually contribute to the disease process or merely reflect ongoing pathology. Recently, however, it has become clear that reactive microglia surround only certain types of amyloid deposits in the brain (the neuritic and core plaques) while ignoring nearby deposits of other types, including diffuse plaques (Perlmutter et al., 1992; Giulian et al., 1995a). Such selectivity in the distribution of reactive glia suggest that specific signals within neuritic and core plaques drive brain inflammation. With the increasing recognition that reactive microglia can mediate neuronal injury through release of cytotoxic factors (Banati et al., 1993; Giulian et al., 1993a), speculation on the involvement of microglia in AD has encompassed the release of complement proteins (Rogers et al., 1988, 1992), cytokines (Meda et al., 1995; Mrak et al., 1995), NMDA-like toxins (Piani et al., 1991; Giulian et al., 1995a, b), and free radicals (Thery et al., 1991; Hensley et al., 1994.
The present invention demonstrates that A.beta.1-42 is the plaque-derived component which elicits neurotoxic responses in microglia. Importantly, the N-terminus of human A.beta. provides an anchoring site necessary for initiating this neurotoxic cascade. More particularly, the HHQK-containing sequence is found to be significant in initiation. Because HHQK-like agents suppress toxic microgliosis in AD brain, neuronal loss and dementia may thereby be slowed. The present invention provides strategies for blocking the activation of microglia, thus making possible screening for therapies for the treatment of inflammatory injury to neurons in conditions such as Alzheimer Disease, stroke, trauma, multiple sclerosis (MS), Parkinson's disease, HIV infection of the central nervous system, amyotrophic lateral sclerosis (ALS), hereditary hemorrhage with amyloidosis-Dutch type, cerebral amyloid angiopathy, cerebral amyloid angiopathy, Down's syndrome, spongiform encephalopathy, Creutzfeld-Jakob disease, and the like.